This invention relates to the field of optical emitters, and particularly to a power controller for a laser diode.
In U.S. Pat. No. 5,396,059, Paul D. Yeates of ATandT Bell Laboratories (xe2x80x9cYeatesxe2x80x9d) describes a similar invention to control OPS output. However, this invention claims several important improvements. These improvements stem primarily from a difference of approach. Yeates""s approach is to use a simplistic model to guess coarsely at the correct CS to provide the correct output power; then ignore the model, measure and use feedback to obtain exactly the right output regardless of any errors in the model. Although this guarantees absolute accuracy, it is also slow. The present invention uses a complex model and calculates the expected CS through interpolation of discrete measured values. The calculated value is expected to be almost exact since it is calculated from the system""s own behavior, using a sophisticated enough interpolation to ensure accuracy. The feedback mechanism, if used, is only used to compensate for slow drift in the laser""s behavior. This invention does not guarantee absolute accuracy, but it can be made to perform arbitrarily well. Also, this invention is much faster, since it does not require a feedback loop. Transitions between power levels can be performed much more smoothly without any need to verify and correct the output power.
The improvements of the present invention are described below:
1. Yeates describes calibrating one prototypical system and storing its parameters in all systems. This invention improves upon this by calibrating each system individually, which provides more accurate parameters for each system.
2. Yeates uses a calibration mechanism with a temperature controller, as this invention does in the exemplary case. However, Yeates stores the Initial Guess CS""s Iij as a function of the temperature commanded by the temperature controller; in other words, Tj is the temperature commanded by the temperature controller instead of the temperature measured by the system. This invention improves upon this because it calculates X as a function of the actual environmental parameters measured by the environmental sensors embedded in the system, the same ones which will be used to measure the environment during the operation of the system. This has three benefits:
a. The environment commanded may be slightly different from the environment affecting the laser, so measuring the environment near the laser will give a better indication of actual environmental influence;
b. This invention automatically compensates for any systematic error in the environmental sensors, since the same sensors are used for calibration as for operation;
c. This invention allows calibration to be performed during operation, if necessary.
3. Yeates describes storing CS values for each power and temperature value. We can call this table I where the elements Iij=I(Pi, Tj). When an output power P needs to be commanded and the temperature is T, the system looks up the i and j such that Pi is the closest power to P of any powers in the set, and Tj is the closest temperature to T of any temperatures in the set. It then uses Iij directly. It then measures the output power and adjusts the CS until the power matches the desired power P. Although this ensures accurate power output, it also can be very slow.
Instead of setting an initial guess and then using feedback to arrive at the final CS, this invention calculates a theoretical final CS value by some form of interpolation from a large parameter vector X. In this way, it does not need to measure the actual output power and adjust the CS until the power matches. This allows the system to be very fast. Although it is open-loop and the output is not guaranteed to exactly match the desired output, improvements #1 and #2 above help ensure accuracy since each laser-sensor system""s characteristics are measured and stored with that system, and X and the interpolation function can be arbitrarily sophisticated in order to meet accuracy needs. If closed-loop accuracy is desired, the system can also implement Enhancement #2, which compensates for drifting.
4. Yeates stores in the system memory parameters indicating the effect of aging on one prototypical OPS; i.e., it introduces laser age as a parameter in its CS model. Because aging effects are highly variable, this invention measures the effect of aging instead, and adjusts the model X accordingly. This ensures that the model is always up-to-date and accurate regardless of the OPS used. It also allows end-of-life detection to be based on the actual performance of the laser, since the extent of aging can be measured by watching for deviations from the normal.